1. Technical Field
This invention relates to composite thin films having fine particles dispersed therein for use as nonlinear optical materials. Theses fine particle dispersed composite thin films are utilized as materials having greater nonlinear optical effect in an optical information field as optical switches, light wavelength converter elements or the like.
2. Background Art
Development efforts have been made on glass and similar materials having semiconductor ultrafine particles dispersed therein since they exhibit greater optical nonlinearity. Also glass and colloidal solutions having metal fine particles dispersed therein are reported to have relatively greater nonlinearity. In these materials, quartz glass, multi-component glass, polymers and the like are generally used as the matrix in which fine particles are dispersed.
Glass containing gold fine particles known as "gold ruby glass" is one of well-known red glasses. This glass is prepared by once melting glass with about 0.01 wt % of gold added, and then heat treating the glass for color generation.
The gold ruby glass was reported to have a third-order nonlinear susceptibility .chi..sup.(3) of about 10.sup.-11 esu which is not necessarily high enough. This is probably because of the low concentration dispersion that gold has a volume fraction of about 10.sup.-5 in glass. Therefore, one possible approach for increasing nonlinear susceptibility is to increase the concentration of gold fine particles. In the manufacture by a melting method, the solubility limit imposes a certain limit in increasing the concentration.
For overcoming this problem, proposals regarding sputtering and ion-implantation methods which are relatively easy to increase the concentration and control the composition were made in Next Generation Industry Support Technology, Proceedings of the Second Optoelectric Material Symposium, P139-147, 1991 and Japanese Patent Application Kokai (JP-A) No. 294829/1991. Au fine particles are dispersed in SiO.sub.2 glass matrix by sputtering or ion-implantation method in the former, and SiO.sub.2 glass thin films and metal fine particles of Au, Ag and Cu grown in island form are alternately stacked in the latter. It is believed that these metal fine particle dispersed glasses develop greater nonlinearity due to excitation of surface plasmon of the dispersed metal fine particles and quantum size effect, leading to high .chi..sup.(3) values.
In these proposals, however, .chi..sup.(3) is about 1.times.10.sup.-7 esu at the maximum in the former Proceedings, for example, and a further improvement in .chi..sup.(3) is desired. And in these proposals, it was not contemplated to significantly shift the resonance absorption position for adjusting the excitation wavelength. In FIGS. 2 and 3 of the former Proceedings, for example, thin films as deposited are heat treated and measured for absorption spectrum, but the resonance absorption position is shifted thereby only about 30 nm. As a result, for gold fine particles, for example, the excitation wavelength is restricted to about 520 to 550 nm which imposes a limit on an available laser, failing to provide a nonlinear optical material for a particular one of light sources.
Like the above-mentioned .chi..sup.(3) value, an additional factor for improving optical nonlinearity is .chi..sup.(3) /.alpha. wherein .alpha. is a coefficient of absorption, which is referred to as a performance index. It is required to increase the .chi..sup.(3) /.alpha. value. Therefore, there is a desire to have a nonlinear optical thin film having increased values of both .chi..sup.(3) and .chi..sup.(3) /.alpha..